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Background: To verify statistically the hypothesis that birthdate represents a risk factor for Congenital Hypothyroidism, as proposed by some European Authors. Moreover, to verify the existence of seasonal cyclic trends in TSH levels., Methods: For the verification of birthdate as a risk factor, we considered retrospectively a data set of all consecutives newborns of Marche Region (Italy) affected by Congenital Hypothyroidism, in the period 1981-1996 (n = 92). For the evaluation of seasonal cyclic trends in TSH, we considered the level of the hormone in all newborns of Marche Region screened in the period 1989-1996. Data were collected from Neonatal Screening Register of Marche Region. All the statistical analyses are based on circular statistics techniques (Rayleigh test, Roger test, estimation of density by kernel method, Mardia test for circular-linear correlation, Maximization of Mean Vector Length)., Results: No cyclic trend was pointed out with regard to Congenital Hypothyroidism; neither seasonal rhythms nor other cyclic patterns of TSH levels were evidenced., Conclusions: Neither the hypothesis of birthdate as a risk factor for Congenital Hypothyroi-dism, nor the existence of seasonal variations in TSH hormone releasing were confirmed.